Stricter and stricter requirements of environmental conservation have led to modern motor vehicles equipped with internal combustion engines being equipped with so-called "engine management systems". These systems ensure that the engine is always operated at an operating point at which, taking into account the respective requirements of the vehicle driver and the external conditions, the best consumption and the lowest pollutant emission occur.
A problem with engine control systems used today lies in that actuation of the throttle valve arranged in the intake port of the engine is as a rule actuated directly by the driver via a linkage connected to an actuating means, for example a foot pedal or a manual actuator, or a control cable. As a result, engine management follows the displacement of the throttle valve by the driver even when the speed of displacement or the respective opening angle of the throttle valve is not optimally adapted to the operating situation in which the engine and with it the motor vehicle find themselves.
Owing to the difficulties arising from direct coupling of the actuating means and the throttle valve which serves as the actuator for the power consumption of the engine, there has been a change-over to separating the mechanical connection between the actuating means and the actuator. Instead of the mechanical coupling, there is then provided a control device which by means of suitable sensors detects the respective position of the actuating means. Taking into account the vehicle user's standards which then arise and the respective operating situation of the engine, a position of the actuator with which the driver's requirement can be fulfilled in an optimal manner is determined.
In such "drive by wire" systems, the actuator (the throttle valve) is actuated by a servo motor which is controlled by the engine management system. By the fact that the respective current position of the throttle valve is compared with the required position by means of suitable sensors, a closed-loop control algorithm of the engine management system here ensures that the throttle valve is moved into the desired position and stays in it.
Owing to the non-linear relationship between the throttle valve position and the respective operating situation, with known engine management systems the position of the actuator is not directly converted to a corresponding position of the control member. Instead, the position of the actuator is allocated a control value which, taking into account the current operating state of the engine or vehicle, is taken from a data field, the so-called "throttle valve field". In this data field are stored, for example in dependence on the speed of the engine and the respective gear speed engaged, control values which ensure that the respective requirement of the driver indicated by a displacement of the actuating means is fulfilled optimally. Control signals corresponding to the control values concerned are then transmitted to the actuating means which displaces the actuator accordingly.
In order to ensure maximum possible reliability in detection of the signals needed for control of the engine, with known "drive by wire" systems the actuator and the actuating means are each allocated at least two sensors. Their signals are compared with each other, assuming that the signals delivered are error-free as long as they match, apart from a tolerance range.
A problem arises with such systems when one of the sensors is defective or when there is a disturbance of signal transmission for other reasons. In such cases under certain conditions it is difficult to establish which sensor is still delivering a reliable signal and which one is actually unusable. This is true particularly if a disturbance that is only irregularly repeated occurs.
To ensure that in the event of a disturbance of the sensors or signal transmission there is no misinterpretation by the control device of the signals received by the sensors, with known engine management systems it is provided that in the event of such a disturbance the idling state of the engine is automatically induced or the engine is completely turned off. Regardless of the respective driving situation in which the vehicle is at the time the disturbance occurs, in such an event the engine is provided with only as much power as it needs in the idling state. This can lead to considerable difficulties particularly if the vehicle is moving at high speed at the moment of the defect, for example during overtaking.